1. Field of the Invention
The present invention relates in general to digital radio communications and, in particular, to a data framing format for transmitting a stream of digital data.
2. Background Art
The transmission of digital voice data over a radio frequency communication link is very often implemented by transferring the digital data as discrete packets or frames of information from a transmitter to a receiver, as opposed to transferring the data as a continuous homogenous stream of data. In some digital communication systems multiple types, or configurations, of data packets each having different characteristics, such as length and data type, are employed. In such a system one packet type may configured for voice, another for command, and yet another to acquire and maintain synchronization between the transmitter and receiver. A system incorporating multiple types of data packets may often experience control problems which typically increases system complexity, and which contribute to other undesirable design and operating characteristics.
Cordless telephone devices designed to the CT-2 specification are representative of a digital radio communication system incorporating multiple types of data packets. In the environment of a cordless telephone conforming to the CT-2 specification, a telephone conversation begins with initiation of the radio link. During the course of a conversation both audio data corresponding to the user's voice and command data corresponding to user pressed keys and background operation of the phone are transferred over the radio link. In such a system the ratio of audio data to command data is quite large. This large ratio results in a limited or diminished ability to control and monitor operation of the phone during an ongoing active conversation. The CT-2 specification adds further complexity by omitting the use of a synchronization field during active conversation which, in turn, necessitates further processing of the data stream in an attempt to maintain frame synchronization.
Accordingly, it is an object of the present invention to provide a single data frame type containing both audio and command data in every frame. It is a further object to include a separate synchronization field in each data frame to simplify synchronization of the data packets.
Other prior art digital communication systems are constructed utilizing data packets having segregated fields for synchronization, voice, command and error-correction in each frame. This approach creates an inherent delay in audio data transmission and processing directly related to the arrangement of these packets. In most of these systems, the segregation of the various types of data (i.e. voice, command, sync) requires extensive buffering to bundle the overall frame for transmission and, later, to unbundle the overall frame upon reception. As these systems become more complex--resulting in increased sub-field sizes--this delay will enviably increase.
Delay in digital communication is undesirable. Studies have shown that delay in a digital audio data path decreases the Mean Opinion Score (MOS)--which is an indication of users' subjective observation of the audio quality for the transmission system. Part of the decreased audio quality observed has been attributed to delay itself which when long enough to be noticeable may cause users to speak simultaneously or think that the other is slow in responding. Moreover, degradation of the audio quality has been further objectively attributed to the introduction of delayed echoes in the audio channel. Notwithstanding the nature of any such effect upon the audio quality of the communication system, audio channel delay is recognized as undesirable.
Accordingly, it is a further object of the present data frame to minimize delay in the audio channel of a digital communication system.
These and other objects of the present invention will become obvious to those of ordinary skill in the art based upon the attached specification, drawings and claims.